Background: More than 124,000 total hip replacement (THR) surgeries are performed annually in the US in an effort to increase mobility and comfort in those having arthritic, deteriorated or injured hips. The most common preoperative diagnosis for THR is osteoarthritis. As indicated in the NIH Total Hip Replacement Consensus Conference, questions remain concerning rehabilitation approaches which yield the best long-term outcomes. No previous investigations regarding the influence of muscle fatigue in THR patients have been identified. Muscle fatigue and endurance can significantly affect mobility1 function and the risk of secondary injury in this patient population. Aim/Hypothesis: This study seeks to evaluate muscle fatigue in arthritis patients who have undergone total hip replacement (THR) surgery as a means to assess rehabilitation status. The primary hypothesis of our study is that THR patients exhibit increased rates and levels of fatigue placing this population at higher risk for falls, joint dislocations and other injuries. Should this hypothesis prove conclusive, rehabilitation evaluation and training strategies should be expanded to address endurance. Methods. A combination of EMG measures and joint kinetic analysis will be utilized to assess THR patient response to mild muscle fatigue. Test conditions will include dynamometer submaximal (80% MVC) isometric and isokinetic exercises. Response of the patient group's operated hip will be compared to the unoperated hip, as well as to a control group. THR subjects will be assessed postoperatively between 4 and 5 months. Self-assessment of physical and social function will also be evaluated using the MOSS-SF 36, HAQ and MAQ. Preliminary Results/Findings: Our preliminary data from a pilot study (10 cases and 10 controls) funded by the Western Pa. Chapter of the Arthritis Foundation indicates that differences exist between cases and controls in muscle fatigue/endurance and self-assessed physical and social function. Across all biomechanical measures THR cases have lower performance or outcome. Statistically significant differences were found between cases and controls in isokinetic performance associated with hip flexion, extension and abduction. Cases also differed significantly from controls in the level of muscle fatigue associated with 80% MVC exercises as measured by median bower frequency shift. We further observed lower isokinetic performance in case operated hips than unoperated hips. Significant differences were found between operated and unoperated isokinetic performance in hip flexion. Self-assessment of THR cases as measured using the MOSS-SF36 was also consistent lower than that of control subjects.